In electrochromic devices, the color or darkness of a composition is changed by an electrochemical reaction caused by application of an electrical potential difference to electrodes in contact with the composition. Electrochromic devices long have been of interest for providing varying displays in display devices of various types, varying transmittance in variable transmittance lights filters, such as windows, and varying reflectance in variable reflectance mirrors.
In a solution-phase electrochromic device, at least one substance, which changes color or absorbance to light in the visible wavelength range in an electrochemical reaction in operation of the device, is in a solution in the device both at equilibrium with no potential applied to cause an electrochemical reaction and when reduced (if the substance is "cathodic," i.e., electrochemically reduced in the device) or oxidized (if the substance is "anodic," i.e., electrochemically oxidized in the device).
One type of solution phase electrochromic device is a single-compartment, solution-phase, self-erasing electrochromic device. U.S. Pat. No. 4,902,108 (hereinafter the U.S. Pat. No. '108), the entire disclosure of which is incorporated herein by reference, describes a class of such solution-phase electrochromic devices. The U.S. Pat. No. '108 also describes solutions for use as the media of reversibly variable transmittance in such devices; "cathodic" compounds, which undergo a color change on electrochemical reduction in such devices; and "anodic" compounds, which undergo a color change on electrochemical oxidation in such devices. In the devices of the U.S. Pat. No. '108, the reduced cathodic compound and oxidized anodic compound remain in solution and react with each other to regenerate the zero-potential equilibrium forms of both compounds. Further, the U.S. Pat. No. '108 describes the use of the electrochromic devices of the Patent as the components of varying color or darkness in various display devices; the components of reversibly variable transmittance in light filters, such as windows; and the components of reversibly variable transmittance in variable reflectance mirrors, such as dimmable, rearview mirrors for motor vehicles.
In a single-compartment, self-erasing, solution-phase electrochromic device, such as those of the U.S. Pat. No. '108, a solution comprising a cathodic compound and an anodic compound is held in contact with two electrodes. Preferably the solution is held as a layer of solution between and in contact with two, substantially planar, parallel, spaced-apart electrode layers. These electrode layers may be substantially transparent layers (such as of indium-tin oxide or fluorine-doped tin oxide) coated on a substantially transparent material, such as glass. With no potential difference between the electrode layers, and the solution therebetween in its zero-potential equilibrium state, the solution can be clear or slightly colored. When a potential difference of sufficient magnitude (which will typically be less than about 2.0 volts and will depend on a number of factors, including the reduction potentials of the cathodic and anodic compounds in the solution) is applied to the electrode layers (and across the solution therebetween), transmittance of light through the device is reduced, and the color of the device may be perceived as changing, as reduction of cathodic compound at the electrode functioning as the cathode yields a composition with a molar extinction coefficient at at least one wavelength in the visible range that is higher than that of the cathodic compound in its zero-potential equilibrium state and, similarly, oxidation of anodic compound at the electrode functioning as the anode yields a composition with a molar extinction coefficient at at least one wavelength in the visible range that is higher than that of the anodic compound in its zero-potential equilibrium state. The rate at which the solution layer darkens or changes color once a potential difference is applied to the electrode layers, and the steady-state darkness or color that results if a potential difference is maintained for a sufficiently long period of time, will depend on a number of factors, including the potential difference, the current across the solution layer, the thickness of the solution layer, and the rate at which reduced cathodic compound is re-oxidized and oxidized anodic compound is re-reduced (e.g., in the case of cathodic compound, by reaction of the reduced cathodic compound with oxided anodic compound in the solution). Once the potential difference between the electrode layers is reduced or eliminated, the color or darkness of the solution layer will return toward its zero-potential equilibrium state, as the concentrations of reduced cathodic compound and oxidized anodic compound will decrease toward lower, steady-state levels. Thus, by adjusting the potential difference between the electrode layers, such a device can function as a "gray-scale" device, with continuous, or step-wise, variable transmittance over a wide range. Ideally, when the potential between the electrodes of such a device is returned to zero, the device spontaneously returns to the same, zero-potential, equilibrium color and transmittance as the device had before the potential was applied.
Approaching this ideal sufficiently closely to make the use of single-compartment, solution-phase, self-erasing electrochromic devices commercially feasible has been a problem. Thus, in most applications of such devices, the cycle-life of the devices (the number of times the potential between the electrodes can be applied and removed without unacceptable degradation in the device, e.g., in the color or darkness of the device in the zero-potential, equilibrium state or the steady state at a selected potential difference) is an important factor and presents a formidable problem. In practical applications, the anodic and cathodic compounds in a solution must be stable to many thousands of cycles of oxidation and reduction.
In applications of single-compartment, solution-phase, self-erasing electrochromic devices in mirrors and windows, a significant problem is stability of the components of the solutions of reversibly variable transmittance in the devices to violet and ultraviolet (UV) light. Thus, when the device is activated (i.e, a potential is placed across the solution of the device sufficient to cause coloration), the performance of the device is typically degraded on exposure of the solution to violet and ultraviolet light. The degraded performance manifests itself as residual color formation in the device, which does not clear up when the device returns to its zero-potential, equilibrium state, and as a decrease in the darkness of coloration attainable with the device when it is activated. Thus high-end (at low or no potential difference) transmission or reflectance of a window or mirror, respectively, is undesirably decreased and the low-end (at significant potential difference with substantially reduced transmittance) transmission or reflectance that can be achieved is undesirably increased.
Under most modes of operation, dimmable rearview mirrors for motor vehicles, which use solution-phase electrochromic devices to provide variable reflectance and their dimming effect, are only colored or dimmed (darkened) at night when light from the headlamps of following vehicles can pose a glare problem for drivers. At night the amount of violet or ultraviolet radiation reaching the solution of the device is insufficient to cause significant degradation over the expected life of the rearview mirror device. However, even in the zero-potential, equilibrium state, solution-phase devices, such as those of the U.S. Pat. No. '108, are slowly degraded by exposure of the solution of the devices to ultraviolet light. This slow degradation process is of particular concern with outside rearview mirrors on motor vehicles since they may be exposed to sunlight containing ultraviolet light day after day over a number of years. In fact, in electrochromic devices which are variable transmittance components of outside, dimmable, rearview motor vehicle mirrors and contain electrochromic solutions according to the U.S. Pat. No. '108 as media of variable transmittance, the electrochromic solutions do degrade over a period of several months to a year to the point where they become unacceptable for outside dimmable rearview mirror use.
Attempts to solve the problem of instability to exposure to ultraviolet ("UV") light have included the use of screens or barriers to UV light, to intercept UV-light before it reaches the solution of the electrochromic device, or the inclusion, in the solution of reversibly variable transmittance in the device, of UV-stabilizing compounds. The UV-stabilizing compounds that have been used have been developed to stabilize organic compositions (e.g., plastics) against degradation by exposure to UV light. The use of screens or barriers of UV-light-absorbing material adds significantly to the cost of materials for electrochromic devices and significantly complicates the manufacture of such devices by requiring that the screen or barrier be incorporated into the device while avoiding undesirable optical effects that may accompany such incorporation. The use of UV-stabilizing compounds in solutions of reversibly variable transmittance in electrochromic devices, while in many cases capable of increasing the useful life of the devices by retarding photodegradation in the solutions exposed to UV light while in their zero-potential equilibrium state, are not totally effective in eliminating this UV-induced photochemical degradation. Further, the degradative products that do form in the solutions when UV-stabilizing compounds are present significantly reduce the useful cycle life of the devices when they are cycled between the clear and colored state, even, e.g., at night, in the absence of light which might cause additional photochemical degradation during cycling. Thus, a problem confronting the art of solution-phase electrochromic devices, including single-compartment, solution-phase, self-erasing electrochromic devices, is to identify compounds which are suitable for use as anodic or cathodic compounds in the solutions of reversibly variable transmittance in such devices and that render such solutions, at least in their zero-potential equilibrium states, highly stable to exposure to ultraviolet light from e.g. sunlight.
There are a number of other desireable characteristics in anodic or cathodic compounds for use in solutions of variable transmittance in solution-phase electrochromic devices, including those which are single-compartment and self-erasing. Such compounds are desirably stable in solution by themselves and in combination with each other in their zero-potential equilibrium states even at high temperatures above about 80.degree. C. and preferably up to about 120.degree. C. The anodic and cathodic compounds in their colored or activated states in solution should be stable at at least 50.degree. C. and preferably at at least 75.degree. C., so that devices containing solutions of the compounds can be cycled without significant degradation between their colorless and colored states at at least 50.degree. C. and preferably at at least 75.degree. C. Anodic and cathodic compounds for media of variable transmittance in electrochromic devices which need to operate over a large range in transmittance or reflectance, as in high quality, dimmable rearview mirrors for motor vehicles, should be colorless or nearly colorless in their zero-potential equilibrium state, undergo a large increase in molar extinction coefficient in the visible range on oxidation or reduction, and be soluble enough to provide for the range of transmittance or reflectance required in the device in which a solution of the compound is employed to provide variable transmittance.
A particularly preferred solution for use as the medium of reversibly variable transmittance in a single-compartment, solution-phase, self-erasing electrochromic device, used to provide variable reflectance in a variable reflectance rearview mirror for a motor vehicle, consists of propylene carbonate as solvent, benzylviologen difluoroborate (i.e., 1,1'-dibenzyl-4,4'-bipyridinium difluoroborate) as cathodic compound at 5 mM -50 mM in the solution at room temperature, 5,10-dihydro-5,10-dimethyl phenazine as anodic compound at 5 mM -50 mM in the solution at room temperature, 1-10% (w/w) polymethylmethacrylate to thicken the solution, and ethyl 2-cyano-3,3-diphenyl acrylate at 200 mM to 700 mM in the solution at room temperature to somewhat retard UV-associated photodegradation in the solution in its zero-potential equilibrium state. This system of compounds fits the stability, color and solubility criteria discussed above. However, for long lived outside rearview mirrors, especially outside rearview mirrors on automobiles, and certain window applications, very high stability against long term exposure to sunlight is required while the device is colorless or unactivated. A problem remains with the above system of compounds: slow degradation in sunlight, with the degradation products that form adversely affecting the cycle life of the devices containing the systems of compounds.
Salts of 4,4'-bipyridinium dications, in addition to the well known viologen salts, derivatized at the 1 and 1' positions are known. See Emmert and Varenkamp, Chemishe Berichte 56B, 491-501 (1923); and Sugiyama et al., Japanese Patent Application Publication No. 61-148,162 (1986). There have been no reports suggesting the use, or the advantages of using, salts of 4,4'-bipyridinium dications derivatized at 1 or 1' position with alkyl, phenyl or benzyl, and at at least one of the 2,2',6 and 6' positions with alkyl as cathodic compounds in solutions of reversibly variable transmittance in solution-phase electrochromic devices.